1. Field of the Invention
The present invention relates to magnetic bearing elements, and more specifically, it relates to a means for combining the functions of a passive magnetic bearing system with that of a generator/motor.
2. Description of Related Art
Klaus Halbach has investigated many novel designs for permanent magnet arrays, using advanced analytical approaches and employing a keen insight into such systems. One of his motivations for this research was to find more efficient means for the utilization of permanent magnets for use in particle accelerators and in the control of particle beams. As a result of his pioneering work, high power free-electron laser systems became feasible, and his arrays have been incorporated into other particle-focusing systems of various types.
In August 1979, K. Halbach authored a paper entitled "Design of Permanent Multipole Magnets with Oriented Rare Earth Cobalt Material." In this paper, he presented a novel method of generating multipole magnetic fields using non-intuitive geometrical arrangements of permanent magnets. In subsequent publications, he further defined these concepts.
The present inventor incorporated some of the concepts studied by Klaus Halbach in an "inside-out" design, where the rotating portion, i.e., the rotor, is on the outside of the stator. See U.S. Pat. No. 5,705,902. The stationary portion, i.e., stator, is formed by the inside of the machine and is inserted down the axis of the dipole field. The cylindrical rotor contains an array of identical permanent magnets that provide a uniform dipole field. The windings of the motor are placed in or on the stator. The stator windings are then "switched" or "commutated" to provide a DC motor/generator much the same as in a conventional DC motor. The commutation can be performed by mechanical means using brushes or by electronic means using switching circuits. The cited invention is useful in electric vehicles and adjustable speed DC drives.
Electric vehicle drive motors need to be rugged, low loss, and easy to control. A DC motor in general is easier to control than an AC motor. The Halbach array allows for the construction of an "ironless" motor thereby eliminating hysteresis and eddy current losses creating a very efficient motor. Efficiencies greater than 95 percent are possible. In conventional motors/generators using iron, the narrowness of the airgap between stator and rotor dominates the machine design. Since the Halbach array provides a uniformly distributed magnetic field and one not requiring the use of narrow-gapped iron pole faces, the size of the airgap is no longer an important design parameter. This allows the motor design to be insensitive to airgap size allowing looser tolerances, which then provides for a less expensive, more rugged design. The more rugged, less expensive design coupled with greatly increased efficiency makes this a nearly ideal design for many applications.
Motor and generator armatures, flywheel rotors, and other rotatable components have conventionally been supported and constrained against radially and axially directed forces by mechanical bearings, such as journal bearings, ball bearings, and roller bearings. Such bearings necessarily involve mechanical contact between the rotating element and the bearing components, leading to problems of friction and wear that are well known. Even non-contacting bearings, such as air bearings, involve frictional losses that can be appreciable, and are sensitive to the presence of dust particles. In addition, mechanical bearings, and especially air bearings, are poorly adapted for use in a vacuum environment.
The use of magnetic forces to provide a non-contacting, low friction equivalent of the mechanical bearing is a concept that provides an attractive alternative, one which is now being exploited commercially for a variety of applications. All presently available commercial magnetic bearing/suspension elements are subject to limitations, arising from a fundamental physics issue, that increase their cost and complexity. These limitations make the conventional magnetic bearing elements unsuitable for a wide variety of uses where complexity-related issues, the issue of power requirements, and the requirement for high reliability are paramount.
The physics issue referred to is known by the name of Earnshaw's Theorem. According to Earnshaw's Theorem (when it is applied to magnetic systems), any magnetic suspension element, such as a magnetic bearing that utilizes static magnetic forces between a stationary and a rotating component, cannot exist stably in a state of equilibrium against external forces, e. g. gravity. In other words if such a bearing element is designed to be stable against radially directed displacements, it will be unstable against axially directed displacements, and vice versa. The assumptions implicit in the derivation of Earnshaw's Theorem are that the magnetic fields are static in nature (i. e. that they arise from either fixed currents or objects of fixed magnetization) and that diamagnetic bodies are excluded.
The almost universal response to the restriction imposed by Earnshaw's Theorem has been the following: Magnetic bearing elements are designed to be stable along at least one axis, for example, their axis of symmetry, and then external stabilizing means are used to insure stability along the remaining axes. The "means" referred to could either be mechanical, i.e., ball bearings or the like, or, more commonly, electromagnetic. In the latter approach magnet coils are employed to provide stabilizing forces through electronic servo amplifiers and position sensors that detect the incipiently unstable motion of the rotating element and restore it to its (otherwise unstable) position of force equilibrium.
Less common than the servo-controlled magnetic bearings just described are magnetic bearings that use superconductors to provide a repelling force acting against a permanent magnet element in such a way as to stably levitate that magnet. These bearing types utilize the flux-excluding property of superconductors to attain a stable state, achieved by properly shaping the superconductor and the magnet so as to provide restoring forces for displacements in any direction from the position of force equilibrium. Needless to say, magnetic bearings that employ superconductors are subject to the limitations imposed by the need to maintain the superconductor at cryogenic temperatures, as well as limitations on the magnitude of the forces that they can exert, as determined by the characteristics of the superconductor employed to provide that force.
The magnetic bearing approaches that have been described represent the presently commonly utilized means for creating a stable situation in the face of the limitations imposed by Earnshaw's Theorem. The approach followed by the first one of these (i.e., the one not using superconducting materials) is to overcome these limitations by introducing other force-producing elements, either mechanical, or electromagnetic in nature, that restore equilibrium. The latter, the servo-controlled magnetic bearing, is usually designated as an "active" magnetic bearing, referring to the active involvement of electronic feedback circuitry in maintaining stability.
The magnetic bearing approach employed in this invention is of a different type than either of the just-described approaches. It is of a type that might be called an "ambient-temperature passive magnetic bearing." This type of bearing, of the type described in U.S. Pat. No. 5,495,221, employs permanent magnet and other elements, together with dynamic effects, to overcome the limitations of Earnshaw's Theorem.